The present invention relates to partially enhanced drill bits for drilling in ground formations as defined by the preambles of the subsequent independent claims.
In percussive drilling in subterranean rock formations, a drill bit is alternatively rotated and impacted, whereby pieces of rock are broken away. The impacts can be generated locally by a down-the-hole piston, or from the ground surface.
A conventional drill bit for percussive drilling comprises a steel body with button inserts mounted on the drilling face. The buttons are arranged in generally concentric annular rows, with at least the outer row known as the gauge row, and the other rows known as the face rows, on larger bits, several outer rows may be referred to as gauge rows. The bit includes flushing holes for conducting flushing fluid that conducts-away the cuttings,
In order to minimize the rate of wear of the buttons, the buttons are usually formed of a hard material such as cemented carbide. The gauge row buttons perform the most work, and thus diamond-enhanced buttons are occasionally used as gauge buttons, i.e., buttons having a diamond layer applied thereto, such as natural or synthetic diamond. Bits having diamond enhanced gauge buttons, and non-diamond enhanced face buttons are called "partially-enhanced" bits. Bits wherein both the face buttons and gauge buttons are diamond-enhanced are called "fully enhanced" bits.
The wearing of the non-enhanced face buttons requires that the bit be pulled from the ground so that the non-enhanced face buttons can be re-shaped. It is necessary to remove some of the steel bit body to expose more of the face button to enable the face button to be re-shaped. Re-shaping is performed a number of times until the face buttons are so worn that the gauge buttons become overloaded and break.
The above-described problem could be addressed by using diamond-enhanced buttons in the face, but this solution greatly increases the cost of the bit.
It would be desirable to alleviate the problem in a more cost-effective way.